`equalize_hist`: "max_intensity" parameter no longer exists---img_as_float normalizes intensity range
`cumulative_distribution`: Return centers of bins instead of the edges.
Move `histogram` function from filter subpackage.
Add test of equalize_hist
Add example of histogram equalization.
This change distinguishes it from the `bins` argument in numpy.histogram, which can accept both the number of bins or a sequence bin edges. Also, this name matches other function parameters in the scikit (e.g. `histograms` in io/_plugins/util.py).